Metricbeat: Kubernetes StatefulSet metricset

StatefulSet is the Kubernetes workload API object used to manage stateful applications. Thanks to a community contribution by @fassisrosa, the Metricbeat Kubernetes module now has a metricset that captures metrics about each StatefulSet. This new metricset is scheduled to be released with Beats 6.3.

Metricbeat: Fix filesystem used percentage

The used percentage reported by Metricbeat does not match what is reported by df. Metricbeat's computation of the system.filesystem.used.pct value was based on the total number of bytes, but df uses the total number of bytes available to the unprivileged user. This is getting fixed via this PR. The fix will be in 6.3.